A theoretical calculation method is reported for obtaining strain and temperature coefficients of fibers with multimode acoustic waveguide structure in distributed optical fiber sensing based on Brillouin beat spectrum power measurement. The power of the Brillouin beat spectrum relates to the fiber’s effective refractive index, Young’s modulus, Poisson’s ratio and acousto-optic effective area, and these fiber’s parameters change with the strain and temperature in the fiber. When the profile of refractive index for a studied fiber is available, the Brillouin beat power-strain coefficients and Brillouin beat power-temperature coefficients of fibers with multimode acoustic waveguide structure are deduced by establishing the relationship between Brillouin beat power and strain and temperature. The large effective area fiber (LEAF) is taken as an example to calculate the strain and temperature coefficients of the LEAF and the results are compared with experimental measurements. The results show that the theoretical calculated values obtained by using this method are consistent with the experimental values, and thus the effectiveness of the proposed calculation method is verified.